Semipermeable membranes for blood treatment based on acrylonitrile copolymers are known. Satisfactory acrylonitrile copolymers are disclosed in U.S. Pat. No. 4,749,619. Such membranes are symmetrical, semipermeable hollow fibers having a homogeneous mesoporous structure and a composition which is substantially uniform throughout their thickness. These fibers comprise a copolymer of acrylonitrile and an olefinically unsaturated comonomer bearing sulphonic groups which may be salified.
The fibers are obtained by lowering the temperature of a polymer solution extruded from a die until a homogeneous gel structure is obtained, followed by a washing operation and then a stretching operation. Hollow fibers produced have remarkable permeability characteristics for dialysis and ultrafiltration, and exhibit excellent biocompatibility.
However, these membranes have some disadvantages. For example, they must be stored in a moist state or be impregnated with a hygroscopic substance having a low vapor pressure such as glycerine. Otherwise, they irreversibly lose their permeability and mechanical characteristics, and thus become relatively useless in hemodialysis.
Asymmetric semipermeable membranes are also known in the medical field. They comprise a hydrophobic polymer and a hydrophilic polymer and are employed for blood treatment. For example, European Patent No. 168,783 discloses a microporous membrane based on a hydrophobic polymer and a water-soluble hydrophilic polymer chosen and employed as a pore-former. The hydrophilic polymer is substantially extracted by a precipitating liquid at the die exit. The remaining hydrophilic polymer is also extractable by an aqueous medium, a process step that is undesirable in medical applications.
Semipermeable membranes based on hydrophobic and hydrophilic polymers have been used in other fields, namely industrial ultrafiltration and micro-filtration. U.S. Pat. No. 4,810,384, for example, discloses a hydrophilic and dryable semipermeable membrane based on polyvinylidene fluoride (PVDF) and a hydrophilic polymer. By way of example of hydrophilic polymers, this patent discloses copolymers of acrylonitrile and monomers containing sulphonic groups. In particular, it discloses copolymers of acrylonitrile and sodium methallylsulphonate, optionally with methyl methacrylate. In accordance with the teachings of the patent, a semipermeable, hydrophilic and dryable membrane is obtained from a homogeneous solution of PVDF and a hydrophilic polymer, which is shaped and then coagulated without losing its characteristics.